1. Field of the Invention
This invention relates to a waterborne polyurethane. More particularly, the invention relates to a waterborne polyurethane useful as a film, the film having improved properties comparable to or even exceeding those of rubber, i.e., a percentage elongation greater than 700%, a tensile strength greater than 3500 psi, a 100% modulus below 450 psi, a 300% modulus below 700 psi, and a 500% modulus below 1500 psi.
2. Description of the Prior Art
Many medical devices, particularly protective products, devices and films such as gloves for medical and industrial applications, contraceptive devices, catheters, balloons, cuffs, wound care, and various tubing are manufactured from materials having elastomeric properties. By "elastomeric properties" it is meant that the substance can be stretched at room temperature to at least twice its original length, and, after having been stretched and the stress removed, returns with force to approximately its original length within a short time.
A material having elastomeric properties is also known as an elastomer--the generic term for a rubber. A rubber is defined as a natural, synthetic, or modified high polymer with elastic properties, and, after vulcanization, elastic recovery. The terminology "rubber" is meant to include both natural rubber and synthetic rubber--denoted cis-1,4-polyisoprene, which has extensively been used in the past in the manufacture of many goods, including the aforementioned protective products, devices, and films. Typically, the rubber is initially available as a rubber latex, i.e., a colloidal suspension of the rubber in an aqueous medium, which is then used in the production of the rubbery material.
Use of a rubber latex to manufacture these articles can be problematic from several different perspectives. Because rubber latex is a natural product, it is subject to inherent variations and inconsistencies, which require compensation during the manufacturing process in order to maintain adherence to process and performance requirements. The rubber latex is also susceptible to bacterial degradation, which requires the manufacturer to periodically clean out the process and scrap such material.
Further, the number of latex allergy incidents has been growing rapidly since 1989. In that year, the FDA received its first reports of patients, who when exposed to natural latex medical devices, died from anaphylactic shock. An estimated 15% of the healthcare worker population and a small percentage of the overall population are now sensitive to natural latex. The immunoglobulin E (IgE) latex allergy can manifest itself in three types of reactions;
Type IV: Delayed hypersensitivity (allergic contact dermatitis) PA1 Type II: Irritation (non-allergic) PA1 Type I: Immediate Hypersensitivity (allergic)
The most common allergic response is type IV, a type of contact dermatitis caused by not only the protein in natural rubber latex, but by the additives which improve its properties. These additives include sulfur and sulfur based chemicals and accelerators, mercaptobenzothiazoles, thiurams, carbamates, and phenylene diamines. The type I is the most serious, and possibly fatal, allergic response.
Because of these past problems associated with the manufacture and use of rubber, polyurethane materials have been substituted for rubber in some applications. Polyurethanes are advantageous because they can be biocompatible, can be formed into films with high water vapor transmission, are cooler to wear, and are oil resistant and do not swell in the presence of body oils.
Use of these prior art polyurethane materials, though, has been problematic for several reasons. For example, many of these materials must be extruded, thermoformed or solubilized in an organic solvent to be shaped for use. Further, many of these prior art polyurethane materials are synthesized using a solvent, which can be detrimental if the resultant polyurethane material is used in applications necessitating a solvent-free polyurethane, i.e., protective devices such as condoms, gloves, and catheters. Thus, both safety and environmental considerations have necessitated a search for a waterborne, water-soluble, solvent-free polyurethane.
Also, these prior art waterborne polyurethane materials have been mechanically deficient in that they have not yielded a film having the requisite combination of high tensile strength, high percentage elongation and low modulus characteristic of natural or synthetic rubber. For example, TABLE I below, shows typical values of tensile strength, percentage elongation, and modulus for natural rubber.
TABLE I ______________________________________ PROPERTY NATURAL RUBBER ______________________________________ Tensile strength, psi 4800 Elongation, % 840 100% Modulus, psi 125 300% Modulus, psi 200 500% Modulus, psi 600 750% Modulus, psi 2700 ______________________________________
Waterborne prior art polyurethane materials, on the other hand, have not yielded the requisite combination of high tensile strength, high percentage elongation and low modulus characteristic of the natural or synthetic rubber set out in TABLE I.
Thus, there is a need for a solvent-free, or low solvent containing waterborne polyurethane, which can be used in the production of a film having a balance of high tensile strength, high percentage elongation and low modulus. The polyurethane film must have elastomeric properties like those attributed to rubber--that is, it must stretch under tension, have high tensile strength, retract rapidly, and give nearly complete recovery to its original dimensions.